Advancements in nanotechnology have led to many exciting designs of new products with superior performance. Among applications in various fields, nanotechnology application in the optical display market is considered to be of particular commercial value. The most important asset of a nanotechnology is its ability to accomplish enhanced performance by controlling a material's composition and morphology at nanometer scales. When the size of phase domains in a composite is made much smaller than the wavelength of light (450˜600 nm), the composite's transparency and clarity can be dramatically improved. Light scattering from domains of different refractive indexes is in proportion to the fourth power of the size to wavelength ratio [i.e. ˜(a/λ)4] and, therefore, could be significantly reduced by decreasing domain size. When the domain size is reduced below about 50 nm, the haze level resulting from scattering becomes negligible. Thus, composite nanotechnology provides opportunity for a new generation of optical products, for example, making highly transparent functional coatings for displaying devices. A composite could be designed to accomplish a special function (antiglare, antireflection, anti-staining, antistatic, etc.) and yet with extremely low haze due to the application of nanotechnology. For example, reference is made to the co-pending applications in the names of one or both (or more) of the present inventors: application Ser. No. 10/514,018, filed Nov. 10, 2004; Provisional Application Ser. No. 60/656,096, filed Feb. 25, 2005; Provisional Application Ser. No. 60/659,097, filed Feb. 25, 2005; PCT Application US06/006239, filed Feb. 23, 2006; PCT Application US06/006240, filed Feb. 23, 2006, the disclosures of which are incorporated, herein, in their entireties, by reference thereto.
Fogging is due to water droplets formed by moisture condensation on a solid surface when its temperature is below the dew point. When a cold surface comes in contact with warm, moist air, tiny water droplets condense on the cold surface. The refractive index differences among water droplets (n˜1.33), air (n˜1) and the substrate (n˜1.45) lead to diffusive light scattering and often causes the surface to become translucent. Because of the much-increased use of air conditioning in high humidity areas, fogging of windows, optical glasses and displaying devices has become a major nuisance to users.
Prior art antifog (AF) technologies typically included two approaches to maintain a surface transparent after moisture condensation. To prevent fogging, one could treat a surface by applying a completely hydrophilic coating to absorb all the water molecules into the coating's interior; or alternatively, one could embed hydrophilic surfactants within an otherwise hydrophobic coating to reduce the water contact angle and to spread condensed moisture from scattered and scattering droplets into a flat film (sheeting), thereby minimizing the transmission loss. Both approaches have their own limitations and shortcomings.
Current water absorbing AF coating systems are normally made from crosslinked or non-crosslinked hydrophilic polymers (Hydrophilic acrylic polymers or copolymers: U.S. Pat. No. 3,865,619, U.S. Pat. No. 5,244,935, U.S. Pat. No. 6,506,446, EP 0399441; Crosslinked Polyvinyl Alcohol: U.S. Pat. No. 4,478,909, U.S. Pat. No. 5,262,475, U.S. Pat. No. 5,075,133, U.S. Pat. No. 6,800,365; Hydrophilic Polyurethane: U.S. Pat. No. 3,821,136, U.S. Pat. No. 5,877,254, US Patent Application Publication 20040137155). Water molecules can easily diffuse into this hydrophilic coating layer, thus, preventing moisture condensation on the substrate surface. It is fairly easy to make, but the absorption capacity is limited by the thickness of the coating. In addition, the slow kinetics of absorption by diffusion may not be sufficient to prevent instant fogging in a high humidity environment. If the absorption capacity is saturated either kinetically or thermodynamically, the coating loses its AF effect. The water entrapped in the coating will also swell the coating layer and make the coating more susceptible to mechanical and chemical damages. Adhesion failure, or even delamination, often occurs when used in a high humidity environment.
These mechanical failures are caused by water adsorption into a coating and the subsequent swelling of the coating resin. The second approach was designed to reduce water adsorption by spreading condensed water droplets into a surface thin film. Inclusion of hydrophilic surfactants in a hydrophobic coating could reduce the contact angle, and spread a water droplet into a flat film, if the surfactants migrate to the top surface during processing (U.S. Pat. No. 3,929,699, U.S. Pat. No. 4,214,908, U.S. Pat. No. 4,609,688, U.S. Pat. No. 5,451,460).
Most current approaches include incorporating mobile surfactants into a coating matrix. When moisture condensation occurs, these surfactants will either migrate or turn around to the top surface to reduce the water-solid interfacial tension and the contact angle. However, due to the required mobility, the surfactants are not chemically bonded to the coating structure. Consequently, the surfactants will be washed off the surface either by repeated uses or cleanings, leading to fade-away AF effects. As such, they are suitable for providing only a temporary, i.e. not durable, AF effect. In addition, this surface is prone to damage and staining. Moreover, the plasticizing effect of the surfactant on the coating surface often makes the coating more vulnerable to abrasions and contaminations.